unc-73 is required for cell migrations and axon guidance in C. elegans and encodes overlapping isoforms of 283 and 189 kDa that are closely related to the vertebrate Trio and Kalirin proteins, respectively. UNC-73A contains, in order, eight spectrin-like repeats, a Dbl/Pleckstrin homology (DH/PH) element, an SH3-like domain, a second DH/PH element, an immunoglobulin domain, and a fibronectin type III domain. UNC-73B terminates just downstream of the SH3-like domain. The first DH/PH element specifically activates the Rac GTPase in vitro and stimulates actin polymerization when expressed in Rat2 cells. Both functions are eliminated by introducing the S1216F mutation of unc-73(rh40) into this DH domain. Our results suggest that UNC-73 acts cell autonomously in a protein complex to regulate actin dynamics during cell and growth cone migrations.

Serotonin modulates behavioral plasticity in both vertebrates and invertebrates and in Caenorhabditis elegans regulates key behaviors, including locomotion, aversive learning and olfaction through at least four different 5-HT receptors. In the present study, we examined the serotonergic stimulation of aversive responses to dilute octanol in animals containing null alleles of these 5-HT receptors. Both ser-1 and mod-1 null animals failed to increase sensitivity to dilute octanol on food/5-HT, in contrast to wild-type, ser-4 or ser-7 null animals. 5-HT sensitivity was restored by the expression of MOD-1 and SER-1 in the AIB or potentially the AIY, and RIA interneurons of mod-1 and ser-1 null animals, respectively. Since none of these 5-HT receptors appear to be expressed in the ASH sensory neurons mediating octanol sensitivity, we identified a 5-HT6-like receptor, F16D3.7(SER-5), that was required for food/5-HT dependent increases in octanol sensitivity. ser-5 null animals failed to increase octanol sensitivity in the presence of food/5-HT and sensitivity could be restored by expression of SER-5 in the ASHs. Similarly, the RNAi knockdown of ser-5 expression in the ASHs of wild-type animals also abolished 5-HT dependent increases in octanol sensitivity, suggesting that SER-5 modulates the octanol responsiveness of the ASHs directly. Together, these results suggest that multiple amine receptors, functioning at different levels within the locomotory circuit, are each essential for the serotonergic modulation of ASH-mediated aversive responses.

In both Caenorhabditis elegans and Drosophila, UNC-73/Trio functions in axon guidance by signaling through the Rac GTPase to regulate cytoskeletal rearrangements necessary for growth cone migrations. Here, we show that the complex C. elegans unc-73 gene encodes at least eight differentially expressed UNC-73 intracellular protein isoforms. Previously reported mutations affecting UNC-73 isoforms encoding the Rac-specific RhoGEF-1 domain cause uncoordinated movement, correlating with defects in axon guidance. Mutations in isoforms encoding the Rho-specific RhoGEF-2 domain, which we describe here, result in L1 stage larval lethality with no associated axon guidance defects. Isoform-specific rescue experiments reveal separate functions for the various RhoGEF-2-containing UNC-73 isoforms, which would not likely be discovered by conventional genetic screening. UNC-73 D1 and D2 appear to function redundantly in pharynx muscle to regulate the rate and strength of pharynx pumping, and in the HSN neurons and vulval muscles to control egg laying. Isoforms C1, C2, E, and F act redundantly within the nervous system to regulate the speed of locomotion. The multiple UNC-73 isoforms containing Rac-and Rho-specific RhoGEF domains therefore have distinct physiological functions. In addition to its previously identified role involving RhoGEF-1 in migrating cells and growth cones, our data indicate that UNC-73 signals through RhoGEF-2 to regulate pharynx and vulva musculature and to modulate synaptic neurotransmission.[Keywords: Rho; synapse; behavior; splicing] Supplemental material is available at http://www.genesdev.org.